1. Field of the Invention
The present invention relates to a liquid ejection head and image forming apparatus, and more particularly, to a liquid ejection head and image forming apparatus in which ejection errors such as nozzle blockages are prevented by causing slight vibration of the meniscus to an extent which does not cause ejection of liquid.
2. Description of the Related Art
In an inkjet type of print head, if there is a long non-ejection period during which no ink droplets are ejected from the nozzles, then ejection errors may arise, such as variation of the ejected ink droplets in the volume, the direction of flight, the speed of flight, and the like, and nozzle blockages, due to drying and increase in the viscosity of the ink nearby the meniscus inside the nozzles. Therefore, the image quality may be degraded. A method is known in which, in cases such as this, the ink of increased viscosity inside the nozzles is expelled by performing preliminary ejection (purging), which is not related to printing. However, there is a problem with methods of this kind in that the ink consumption is high.
Therefore, methods have been disclosed for reducing the increase in the viscosity of the ink inside the nozzles, by performing slight vibration of the meniscus to an extent which does not cause ejection of ink droplets from the nozzles (in other words, pulsation of the meniscus) (see, for example, Japanese Patent Application Publication Nos. 2005-95746, 2004-262237 and 2004-202707).
However, the ink inside the nozzles is not churned sufficiently, simply by causing slight vibration of the meniscus, and hence it may not be possible to reduce the increase in the viscosity of the ink.
FIG. 23 shows an example of a print head in the related art, in which the upper part shows an enlarged cross-sectional diagram of the periphery of a nozzle and the lower part is a plan diagram viewed from the side of the nozzle. As shown in FIG. 23, a nozzle flow channel 160 provided in the print head in the related art has a cylindrical section 160a and a tapered section 160b, and a nozzle 151, which is an opening section, is formed on the side of the tapered section 160b. The cross-section perpendicular to the axial direction of the nozzle flow channel 160 is circular at all positions along the axial direction, and the cross-section therefore has a congruent or similar shape with axial symmetry. As shown in FIG. 24, when ink is not being ejected, the meniscus is oscillated (caused to vibrate slightly) upward and downward to an extent which does not cause ejection of an ink droplet, thereby reducing the increase in the viscosity of the ink inside the nozzle flow channel 160. FIG. 25 is an oblique diagram which shows a three-dimensional representation of the internal structure of the nozzle flow channel 160. In FIG. 25, taking the internal diameter of the nozzle flow channel 160 (cylindrical section 160a), to be d, taking the viscosity coefficient of the fluid (namely, ink) flowing inside the nozzle flow channel 160, to be v, and taking the average flow speed of the fluid to be u, then the Reynolds number R (=ud/v) which indicates the state of flow of the fluid can be found. If the Reynolds number R becomes greater than the critical Reynolds number Rc(=2310), then the flow becomes turbulent, and conversely, if it is smaller, then the flow is a laminar flow. If it is possible to create a turbulent flow in the ink inside the nozzle flow channel 160 during vibration of the meniscus, then the ink is churned and hence it is possible effectively to reduce the increase in the viscosity of the ink. However, generally, in an inkjet type of print head, the internal diameter d of the nozzle flow channel 160 is small and it is difficult to make the Reynolds number R greater than the critical Reynolds number Rc. For example, in an inkjet apparatus using water-based ink, the kinematic viscosity of the ink is substantially equal to that of water, at 0.013 cm2/sec, and if d=0.1 mm, then the average speed u required in order to achieve the critical Reynolds number Rc is approximately 30 m/sec, whereas the actual average speed u is approximately 15 to 20 m/sec. Therefore, a turbulent flow cannot be obtained simply by vibrating the meniscus, and the ink inside the nozzle flow channel 160 cannot be churned satisfactorily.
Furthermore, in the tapered section 160b of the nozzle flow channel 160, compressive forces toward the axis of the nozzle flow channel 160 act onto the ink. Theses forces change isotropically in the axial direction, in other words, they simply increase gradually toward the nozzle 151, and they do not allow the ink to be churned satisfactorily.
As described above, in the print head in the related art, it is not possible to reduce increase in the viscosity of the ink effectively. In particular, in a single-pass type of print head, if printing continues over a long period of time, it is difficult to perform preliminary ejection frequently in comparison with a shuttle scan type of print head, and therefore it is necessary to improve the churning effect of the ink during the vibration of the meniscus.